A friction-stir joining is a process wherein a joining tool rotating at a high speed is brought into contact with workpieces or objects to join or weld together the same with resulting friction heat. In this method, there is a fear that a fatigue crack may develop in scars or remains left at a joining start point where the joining tool is placed against the to-be-joined workpieces and at a joining end point where the joining tool is removed from the to-be-joined workpieces.
An example solution to such a problem, namely, positioning a joining start point and a joining finish point away from a joining center line, is proposed in Japanese Patent No. 4436099. According to the proposed solution, a friction stir welding (FSW) start portion is selected at a position laterally spaced a predetermined distance from a joining start point on a joining center line, whereafter joining or welding is carried out from the selected FSW start portion all the way past a joining origin (joining start point) to a joining terminus (joining finish point). Since the joining origin is positioned away from the FSW start portion, there is no fear of a fatigue crack being developed at the joining origin.
In recent years, the to-be-joined workpieces are often plated or coated. Platings and coatings on the workpieces' mated surfaces (joined interface) bring about adverse effects on the resulting joining quality. Certain cases have been observed in which workpieces joined together in accordance with the proposed solution have joining defects at weld origins in the mated surfaces of the workpieces. Thus, there is a demand for an improved joining method that achieves sound joining of plated or coated to-be-joined workpieces, particularly at joining origins of the latter.